Scalaz Reader to ReaderT - scala

Question

A function which I cannot change returns Scalaz Reader,
type Action[A] = Reader[Session, A]
def findAccount(s: String): Action[Account] =
Reader((session: Session) => Account(s))
I want create a new function based on findAccount(...) to return ReaderT[Option, Session, A] as in
type ActionT[A] = ReaderT[Option, Session, A]
def findAccountT(s: String): ActionT[Account] = findAccount(s).map(Option(_))
because eventually I want to do this,
def findAccBalT(accountNumber: String) = for {
acc <- findAccountT(accountNumber)
bal <- findBalanceT(acc)
} yield bal
How do I proceed? Does it make sense? Thanks
Full disclosure,
import scalaz._
import Scalaz._
trait Session {
def doSomething(): Unit
}
case class Account(number: String) extends AnyVal
case class Amount(value: Int, currency: String)
case class DBSession() extends Session {
override def doSomething = println("writing to db")
}
type Action[A] = Reader[Session, A]
type ActionT[A] = ReaderT[Option, Session, A]
def findAccount(s: String): Action[Account] =
Reader((session: Session) => Account(s))
def findBalance(account: Account): Action[Amount] =
Reader((session: Session) => Amount(333, "S$"))
// failed
def findAccountT(s: String): ActionT[Account] = findAccount(s).map(Option(_))
// failed
def findBalanceT(account: Account): ActionT[Amount] = findBalance(account).map(Option(_))
// failed
def findAccBalT(accountNumber: String) = for {
acc <- findAccountT(accountNumber)
bal <- findBalanceT(acc)
} yield bal

Answer 1

Short answer: You can use mapK.
Reader[A] is a type alias for ReaderT[Id, A] and ReaderT is an alias for Kleisli. Id[A] is the same as A.
In the Kleisli ScalaDoc we find mapK :
def mapK[N[_], C](f: (M[B]) => N[C]): Kleisli[N, A, C]
Since we know that Reader[Session, A] is the same as Kleisli[Id, Session, A] we can use mapK to go to Kleisli[Option, Session, A] :
import scalaz._, Scalaz._
type Session = String
type Action[A] = Reader[Session, A]
type ActionT[A] = ReaderT[Option, Session, A]
val action: Action[String] = Reader(s => s)
val actionT: ActionT[String] = action mapK Option.apply